The present invention is directed to a screw retainer for a bone plate.
Back pain is one of the most common and often debilitating conditions affecting millions of people in all walks of life. Today, it is estimated that over ten million people in the United States alone suffer from persistent back pain. Approximately half of those suffering from persistent back pain are afflicted with chronic disabling pain, which seriously compromises a person's quality of life and is the second most common cause of worker absenteeism. Further, the cost of treating chronic back pain is very high, even though the majority of sufferers do not receive treatment due to health risks, limited treatment options and inadequate therapeutic results. Thus, chronic back pain has a significantly adverse effect on a person's quality of life, on industrial productivity, and on heath care expenditures.
Degenerative spinal column diseases, such as disc degenerative diseases (DDD), spinal stenosis, spondylolisthesis, and so on, need surgical operation if they do not take a turn for the better by conservative management. Typically, spinal decompression is the first surgical procedure that is performed. The primary purpose of decompression is to reduce pressure in the spinal canal and on nerve roots located therein by removing a certain tissue of the spinal column to reduce or eliminate the pressure and pain caused by the pressure. If the tissue of the spinal column is removed the pain is reduced but the spinal column is weakened. Therefore, fusion surgery (e.g., ALIF, PLIF or posterolateral fusion) is often necessary for spinal stability following the decompression procedure. However, following the surgical procedure, fusion takes additional time to achieve maximum stability and a spinal fixation device is typically used to support the spinal column until a desired level of fusion is achieved. Depending on a patient's particular circumstances and condition, a spinal fixation surgery can sometimes be performed immediately following decompression, without performing the fusion procedure. The fixation surgery is performed in most cases because it provides immediate postoperative stability and, if fusion surgery has also been performed, it provides support of the spine until sufficient fusion and stability has been achieved.
Conventional methods of spinal fixation utilize a rigid spinal fixation device to support an injured spinal part and prevent movement of the injured part. Among these conventional spinal fixation devices are plates configured to be positioned adjacent to the injured spinal part, and secured to the bone by screws. An increasingly accepted procedure for treating spinal disorders involves using the rigid plates to hold vertebrae in desired spatial relationships and orientations relative to each other. The upper cervical spine can be approached anteriorly or posteriorly. In either case, holes are drilled and tapped in at least two of the vertebrae, to receive screws or other fasteners used to secure the plate. The holes are accurately positioned with reference to openings formed through the cervical plate. In some cases the screws may be self-tapping. Typically the plate is curved about its longitudinal axis to facilitate contiguous surface engagement of the plates with the vertebrae. With the plate maintained against the vertebrae, the fasteners are secure within the holes. As a result, the plate maintains the attached vertebrae in a desired spacing and orientation with respect to each other.
One of the problems associated with this technique is the tendency of screws or other fasteners to gradually work loose after fixation. Slight shock or vibration of the vertebrae, due to walking, climbing stairs or more vigorous activity by the patient following treatment increases this tendency, jeopardizing the integrity of fixation. Moreover, as the fasteners work loose, the outward protrusion of the heads over other components of the fasteners can be a source of discomfort and present the risk of trauma to adjacent and surrounding soft tissue.
The curvature of cervical plates typically results in a convergence of fasteners that extend through spaced apart openings in the plate, particularly when each screw is perpendicular to the region of the plate surrounding it. Screws sufficiently short to avoid interfering with one another may not be long enough to assure a secure plate fixation. Further, the physician may encounter difficulties in positioning the plate if one of the vertebrae, due to a particular shape and orientation, cannot readily retain a perpendicularly inserted fastener.
There have been attempts to solve the above problems. For example, screws or other fasteners can be provided with somewhat rounded heads as shown in U.S. Pat. No. 5,324,290. U.S. Pat. No. 5,261,910 shows a nut with a rounded upper surface and a hexagonal recess. The nut has a conical portion that fits into a similarly shaped recess in a plate, thus to reduce the height at which the nut extends above the plate. U.S. Pat. No. 5,364,399 discloses an anterior cervical plate system in which the openings through the plate guide the screws in a non-perpendicular orientation that causes the screws to diverge, rather than converge, as they proceed into the particular cervical vertebra. The plate has a recess for limiting the degree of outward protrusion of the screw heads. After each pair of screws is fully inserted, a locking screw is threaded into the plate until its head encounters the heads of the bone screws.
U.S. Patent Publication No. 2005/0021032 discloses a cervical spine fixator including screw head fixture members having a protruder, which is deformed under the effect of pressure by a screw head when the screws are inserted and returns to their position of origin when the screw head is completely accommodated into the plates. The plates include receiving grooves communicating with the respective apertures, where the grooves are for accommodating the screw head and the screw head fixture member. Among the disadvantages of this design is the weakening of the plate in the area of the apertures due to the receiving grooves. This weakening may cause failure of the device under loading conditions.
U.S. Pat. No. 6,626,907 discloses an anterior cervical plate that uses threaded fasteners through respective apertures for securing the plate to vertebrae. The cervical plate incorporates a fastener retaining feature in the form of a cantilevered tab or a beam supported at its opposite ends, and plastically deformable between an open position for admitting the fastener and a closed position for preventing retraction. A disadvantage of this approach to securing the screws is again the requirement for a recess or groove in the area of the aperture to accommodate the deformable retaining feature. Further, as the retaining feature must be bent (past its point of elasticity) there is the possibility of breaking the retaining feature during the surgery. Depending on when the breakage occurs, this might require the removal of a number of screws that have already been tightened down, a very undesirable necessity. The screw removal problem is exacerbated by the retainer feature itself, as it cannot be easily reversed. Indeed, once deformed, the cold work of the material makes it more difficult to reverse the deformation process to remove the screws.
Therefore, conventional devices for screw retention have not provided a comprehensive, simple, and reliable means of counteracting the tendency of the bone screws to work loose after cervical plate fixation.